[gdb/testsuite] Fix testing build_executable result

[gdb/testsuite] Fix testing build_executable result

When running with target board unix/-feliminate-dwarf2-dups, we run into these
FAILs:
...
FAIL: gdb.cp/rvalue-ref-params.exp: print value of f1 on Child&& in f2
FAIL: gdb.cp/ref-params.exp: print value of f1 on Child in main
FAIL: gdb.cp/ref-params.exp: print value of f2 on Child in main
FAIL: gdb.cp/ref-params.exp: print value of f1 on Child& in f2
FAIL: gdb.cp/ref-params.exp: print mf1(MQ)
FAIL: gdb.cp/ref-params.exp: print mf2(MQ)
FAIL: gdb.cp/ref-params.exp: print f1(MQR)
FAIL: gdb.cp/ref-params.exp: print mf1(MQR)
FAIL: gdb.cp/ref-params.exp: print mf2(MQR)
...

This is due to comparing the result of build_executable to 1, while
build_executable returns either 0 for success, or -1 for failure.

Fix this by comparing with -1 instead.

Tested on x86_64-linux.

gdb/testsuite/ChangeLog:

2020-03-07  Tom de Vries  <tdevries@suse.de>

	* gdb.cp/ref-params.exp: Compare build_executable result with -1.
	* gdb.cp/rvalue-ref-params.exp: Same.